- 4 Claims. c1. 260-68333) The present invention relates to improvements in the art of isomerizing paraflinic naphtha fractions to improve octane quality. More particularly the invention concerns a novel process forpreparing the feed stream of an isomerization reaction to improve the efficiency thereof and involves the use of an adsorption material selective to aro- I 2,937,215 Patented May 17,1960

naphtha isomerization step as well as to upgrade low octane normal and isoparaffins to high octane branched chain paraifins. I

In accordance with the present invention the removal of aromatics fromthe isomerization feed is accomplished by contacting the feed with an adsorbent material selectiveto aromatic hydrocarbons, preferably with one of a "group of substances known as molecular sieves. A novel of recovering the adsorbed aromaticsvary in diameter from less than 3 or 4 to 15 or more matic hydrocarbons. Preferred adsorption materials are a group of substances known as molecular sieves. 4

With the modern trend to the use of higher compression ratios in automotive engines there has been an in:

creasing demand for rnotorfuels of higher octanerating.

One of the'processes employed'for, improving the octane quality of naphthas that are blended into gasoline in-.

vol ves subjection of a fraction comprising normal paraffinhydrocarbons to a catalytic isomerization reaction wherein conversion to branch chain hydrocarbons takes place. The isomerization .Ofnormal paraillns to isoparaffins in the presence of a Friedel-C'rafts type catalyst, of

which aluminum chloride is a typical example, and in the presence of promotional amountsof halogen-containing promoters, of which hydrogen chlorideis a typical example, is old. Numerous processeshave been devised, both vapor phase and "liquid phase, for the isomerization of normal paraffins to the corresponding isop arafiins.

When the light naphtha that is being isomerized contains appreciable amounts of aromatic hydrocarbons such as benzene, the efiiciency of the isomerization reaction is seriously impaired. The presence of benzene in the feed to the isomerization reaction in quantities greater than about 1.0 volume percent is injurious to the product quality while smallquantities of benzene inconcentrations of about 0.2 to 0.5 percent are beneficial. To ensure that the isomerization reaction will not be retarded, it is necessary to remove these aromatic hydrocarbons or substantially reduce theirconcentrationi'in the feed; stream. 1 Inaddition the presence of benzene interferes with the separation of heavy naphthenicbottoms by fractionation,

which'is a desirable step in preparing the light naphtha as an isomerization feed. The benzene, which has anormal boiling point of 176 F., forms low boiling azeotropeswith normal hexane and naphthenes such asinethylc'yclop'entane and cyclohexane. Efiicient separation of the naphth enes and benzene from the parafiinic compounds is impossible because of the azeotropes which tend tocome overhead witliithe' desirable paraffinic com- 7 pounds. These azeotropes boil in the same range as'does normal hexane'in a light naphtha cut, i.e. 150 to 160 F.

-Once thebenzene'is removed, this separation becomes e simple. The separation of aromatics can be done by such 'methods as'solvent extraction, extractive distillation, and

low pressure hydrogenation using a catalystsuch as palladiur'n. Operations of this type are costly, however, and

the: need for better methods forremoving aromaticshas continued tolexist. v l a 7 "It is the principalobj'ect of the present inventidn to provide a more efiicient method for removing benzene and related aromatic hydrocarbons from the feed to a ties, such zeolites areknown as molecular sieves. Certain synthetic zeolites also have molecular sieve properties as taught; for example, by Barrer in U.S. Patent 2,306,610 and by Black in U.S. Patent 2,442,191.'

In practicing the present invention, molecular sieves of from about 6 to 15 Angstrom size are preferred. Molecular sieves of this-range of pore sizes are very highlyselective to aromatic hydrocarbons over other compounds found in light naphtha. For example, a 13 Angstrom sieve may be effectively employed. Such a sieve may be prepared by reaction of a sodium silicate having a high ratio of sodium to silica, e.g. sodium metasilicate with a sodium aluminat e having a soda-to-alumina ratio offromlzl to 3:1, the proportion of sodium silicate solution to sodium aluminate solution being such that the ratio'of silica-to-alumina inthe final mixture is at least 3:1 and preferably from about 4:1 to about 10:1. 'Preferably the sodium aluminate solution is added to the sodi um metasilicate solution at ambient temperatures while employing rapid and eificient agitation so as to ensure the formation of a precipitatehaving an essentially uniform composition throughout. The resulting homogeneous paste is heated to about 180 to 215 F. for a period as long'as200 hours or more to ensure that the crystals therebyiformed will have the desired pore size of about 13"Angstroms. After the period of heat soaking, the precipitated sodium alumino-silicate is filtered and water Washed and then dried and activated in a calcining zone ing..

'-:--.Fig.v 1 schematically'shows a fiow'plan of a process that is particularly adapted for practicing the invention when the feed has a high initial concentration of di- Fig. 2 is an alternate flow plan of the same nature as *Fig; 3 is a flow plan of a process particularly useful for feeds having a low initial concentration of dimethylbutane.

Referring to Fig. 1 in detail, a light naphtha boiling in the range of 60 F. to about 180 F. and containing benzene and normal paraflinic hydrocarbons, and which I may also contain isoparaffinic hydrocarbons and naphthenic hydrocarbons, is supplied to the system through n'aphthafeed line 11. The feed is first preferably frac- 'tionate d in the fractionating vessel 12 which is usually designated as a -depentanizer since his adapted to *remove from the feed all hydrocarbons that boil below the range of 6 carbon atom hydrocarbons. These lower boiling hydrocarbons leave the vessel through line 15.

The depentanized feed is then charged through line 16 into a molecular sieve treatment zone 18 in either the liquid phase or vapor phase. It is usually preferred to employ vapor phase treatment to ensure high rates of adsorption and to minimize residual feed in the void space of the sieve bed. Vaporization of the feed may be effected either by reducing the pressure or by raising the temperature.

The molecular sieve adsorbent, having'pore diameters of about 13 Angstroms, for example, is arranged in any desired manner in the adsorption zone 18. It may for example be arranged on trays or simply packed within the zone or tower with or without supports. In place of a fixed bed operation, other techniques such as moving beds or fluidized beds may be employed. Where fixed bed operation is used, two or more adsorbent towers are operated in a cyclic manner to ensure continuous processing. Conditions maintained during the molecular sieve treatment in the contacting zone 18 include liquid flow rates of 0.0Sto v./v./hr., temperatures of about 180 to 400 F. and pressures from 0 to 100 p.s.i.g.; With molecular sieves of the indicated size, the benzene in the feed is preferentially adsorbed, while the paraflinc hydrocarbons and naphthenic hydrocarbons, which are for the most part not adsorbed, leave the treating zone through line 19 and are conducted through that line to a fractionating zone 20.

The fractionating zone 20, which is referred to as a superfractionator, is operated under conditions that will remove very high octane doubly branched isoparaffins such as dimcthyl butane overhead through line 21, while normal parafiins and single branched parafiins such as normal hexane and methyl pentanes, as well as naphthenic hydrocarbons, will remain and will be removed through line 22. V

The stream passing through line 22 is conducted to .a rerun tower 24 wherein normal hexane and the methyl pentanes are removed overhead through line 25, leaving behind heavy naphthenic bottoms 'whizh are removed through line 26. A good separation in the rerun tower 24 can only be accomplished if benzene has been previously removed. The presence of benzene in the hexane fraction makes separation difiicult because of the formation of low boiling azeotropes with normal hexane, methyl cyclopentane and cyclohexane, hydrocarbons found in this fraction. If benzene were present, it would be taken overhead into the isomerization reactor 27, where its presence in excessive quantities is injurious. A good separation is only possible if the azeotrope is broken as is done in the present invention by removing benzene with a .molecular sieve. Once benzene is removed, the required separation becomes relatively simple. The benzene-free normal hexane and methyl pentanes in line 25 are sent into an isomerization zone 27 wherein isomerization is conducted in the following manner.

The isomerization reaction is preferably carried out in the liquid phase in the presence of aluminum chloride catalyst supported on Porocel. It is assumed that normal feedstocks contain less than 0.2% olefins. Feeds containing more than this amount may be treated by acid Washing, hydrofining, etc. to reduce the olefin concentration or remove them completely. The conditions .maintained during isomerization in'the reaction zone .27 include flow rates of about 0.3 to 3.0 v./v./hr., preferably 0.5 to 1.0 v./v./hr., temperatures of about 100 to 275 F., preferably 150 to 225 F., and pressures of about 100 to 450 p.s.i.g., preferably 150 to 250 p.s.i.g. The reaction is activated by about 0.1 to 6.0 wt. percent HCl, preferably 1.0 to 2.0 wt. percent H01, and by about 0.1 to 3.0 mol percent hydrogen, preferably 1.0 to 1.3 mol percent hydrogen. Side reactions are reduced and increased product yields are obtained by the addition of about 0.2 to 0.5 vol. percent benzene. Larger quantities retard the isomerization reaction. About 5 to 20 vol. percent naphthenes, preferably 10 to 15 vol. percent naphthenes are also desirable to reduce cracking and prevent catalyst sludging. The naphthenes may be composed of methyl cyclopentane or cyclohexane or mixtures of the two. The naphthene concentration can be maintained by by-passing material from line 26 through line 32 to the isomerization reactor until the desired concentration is obtained. Methyl cyclopentane can also be included in the isomerization feed by cutting the overhead fraction in rerun tower 24 at a higher temperature.

It is not necessary for the molecular sievev to 'be 1 00% eificient since from about 0.2 to 0.5 vol. percent benzene is desirable in the isomerization feed. Benzene addition can be maintained by by-passing material from line 30 through line 33 to the isomerization reactor until the proper benzene concentration is obtained.

The isomerization product is sent back as a recycle through line 28 to the fractionation zone' 20 wherein isoparaffinic materials such as dimcthyl butane are removed overhead through line 21 while non-isomerized normal hexane and partially isomerized methyl pentanes will be recycled to the isomerization step through lines 22 and 25.

The benzene adsorbed on the molecular sieve may be .clesorbed by conventional techniques such as heating,par-

tial pressure reduction, by mechanical means or with purge gases, displacement agents such as olefins or steam, and by combinations of conventional techniques. However, in accordance with the present invention the isoparafiin product streamin line 21 is employed as the desorbing agent. When it is desired to desorb the aromatics from the molecular sieve material in zone 18, the flow of naphtha feed through line 16 is interrupted and is transferred to an alternate zone identical to zone 18. At least a portion of the product stream from line 21, containing principally doubly branched isoparafiins such as dimcthyl butane, is conducted through line 29 into zone 18. The material entering through line 29 is in the vapor phase and has a temperature of F. to 500 F., preferably 140 F. to 200 F. This isoparaffin stream displaces adsorbed benzene and is in turn partially adsorbed on the molecular sieve. The desorbed benzene leaves with the (unadsorbed) isoparaflins through line 30. The benzene, being of high octane quality, enhances the value of the isoparaffin stream as a gasoline component. The use of product streams for desorption has the great advantage of requiring no secondary separation as does the use of external desorbents.

During the adsorption step in vessel 18, the benzene being adsorbed displaces the adsorbed isoparafiins, which are eventually removed overhead in tower 20 through line 21. Extraneous desorbents, such as heavier paraffins and naphthenes, or purge gases, can be used in conjunction with the product d'esorbent for moreefficient gimoval of benzene. These are introduced through line In place of, or in addition to the isoparaflin stream of line 21, the product obtained by isomerizing the pentane out going overhead through line 15 can be used for desorption.

Fig. 2 schematically shows an alternate flow plan :to that of Fig. 1, wherein the depentanized material from vessel 12 is conducted to superfractionator 20 thro gh line 36, so that doubly branched isoparaffins will be removed overhead through line-21 before the feedis sub jected to the molecular sieve treatmentin zone .18, line 37 conducting the feed from fractionator 2.0 to the sieve zone, and line 39 conducting the sieve-treated material to rerun tower 24. In this alternate flow plan the .116- cycle line 28 carries the isomerizationproduct back to line 36 to be handled by the superfractionation tower 20. I

In Fig. 3 is shown another alternative arrangement which is particularly adapted for feeds having a low initial concentration of dimethylbutane. This flow plan differs from that of Fig. 1 in that the superfractionator -is placed after the isomerization zone and thus handles only the isomerization product rather than a mixture of that product with the eflluent from the molecular sieve treating zone. The material not removed overhead from tower 20 is conducted through line 38 to be combined with the efliuent from sieve tower 18 as the feed to the rerun tower 24.

It is to be understood that the specific embodiments of the invention herein described are not intended to limit the invention in any manner. Although the processes have been particularly described with respect to their use of molecular sieves as the adsorbent material selective to aromatic hydrocarbons the processes are also applicable to processes using certain chars which are also selective to aromatic hydrocarbons. Particularly adaptable are the chars known as Columbia G and Columbia L.

Thisinvention is to be limited only by the appended claims.

What is claimed is:

1. A process for upgrading a light naphtha fraction containing aromatic hydrocarbons and normal paraffin hydrocarbons, including normal hexane, which comprises the steps of removing from the naphtha all hydrocarbons lower than C hydrocarbons, passing the remaining fraction through a contacting zone containing an adsorbent material selective to aromatic hydrocarbons, whereby aromatic hydrocarbons are selectively removed from the said naphtha fraction by adsorption, subjecting the resulting aromatic-free naphtha to a fractionation step wherein normal hexane and methyl pentanes are recovered from said last-named fraction, subjecting the normal hexane and methyl pentanes to an isomerization treatment under conditions effecting the production of dimethylbutane, recovering dimethylbutane from the products of the isomerization treatment, interrupting the passing of a naphtha fraction through said contacting zone, conducting at least a portion of said' dimethylbutane through said contacting zone whereby to desorb aromatic hydrocarbons from said adsorbent, and removing a mixture of aromatic hydrocarbons-and dimethylbutane from said contacting zone.

2. Process as defined by claim 1 wherein said adsorbent is a molecular sieve material having pore diametersin the range of about 6 to 15 Angstroms.